A post on HN a couple years ago discussed research showing antidepressants only work for about 15% of patients: https://news.ycombinator.com/item?id=37671529
The thing is, they work very well for that 15%. I suspect the eventual conclusion will be that depression is a syndrome with multiple causes rather than a single condition, and SSRIs treat one of the causes.
Edit: Mark Horowitz is one of the authors of both studies.
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